IP applications such as VoIP (voice over IP) and PWE3 (pseudo wire emulation edge to edge) are highly desirable to have the packet loss with less than 10s of milliseconds during network elements failure. Currently there are various approaches in practice or proposed to speed up the recovery from such failures as discussed below.
For example, MPLS (multi-protocol label switch) LSP (label switched path) fast reroute (FRR) mechanism is used to quickly re-route the RSVP (resource reservation protocol) LSP traffic onto a detour or bypass LSP when a local link failure is detected. Since the detour or bypass LSPs are pre-built before the local link failure, this re-route operation can be accomplished within 10s of milliseconds. If the IP backbone deploys network wide MPLS TE (traffic engineering), this MPLS FRR approach may be a desirable solution. The FRR is just another application using the existing MPLS infrastructure. However, it may be too expensive in certain circumstances to maintain such a network.
In addition, IGP (interior gateway protocol) fast convergence is another mechanism in reducing the packet loss time in network element failure. This mechanism also includes the improvement of LDP (label distribution protocol) convergence. Comparing with the MPLS LSP FRR solution, the recovery time is usually an order of magnitude higher, which is in the 100s of milliseconds range. For certain real-time applications, that duration is still acceptable and this is an improvement over “normal” IGP convergence time of seconds or even 10s of seconds.
Furthermore, pre-calculated alternative nexthops are downloaded into forwarding engines. As in the first mechanism, when a local link failure is detected, those alternative nexthops are used to continue forwarding the data traffic. If an alternative nexthop does exist, then the re-route time can be accomplished within 10s of milliseconds. There is a couple of shortcoming of this approach. There may not exist such an alternative nexthop for the IP destinations along with the links it intends to protect. When such alternative nexthops do exist, if there are many IGP interfaces and adjacencies on the node, this requires running many instances of SPF (shortest path first) in order to find a loop-free alternative. This scheme cannot be used to protect MPLS TE LSPs since they are not constructed from the native IP routing. Further, if the local link failure is shortly after some network events and the IGP on the node is busy calculating those SPFs, then the alternative nexthop picture is incomplete at that time and the re-route action may not be reliable.